Log-periodic antenna



June 21, 1966 R. i.. TANNER Loa-mamme ANTENNA Filed April l1, 1962 5 Sheets-Sheet 1 22h BY ATTORNEY June 21, 1966 R. L. TANNER 3,257,661

LOG-PERIODIC ANTENNA Filed April 1l, 1962 5 Sheets-Sheet 2 22h l" 5 69/ 22C Ej /L/ 22d 7o 22e g 22f 22:22h a 7' 22f INVENTOR. ROBERT L. TANNER uwkgxwm ATTORNEY June 21, 1966 R. 1 TANNER 3,257,661

LOG-PERIODIC ANTENNA Filed April 1l, 1962 5 Sheets-Sheet 3 Fig 7 /W//N//N/N/N/@/N/N//w/N/N/N/N/N//N INVENTOR. ROBERT L. TANNER BY Mgwv .lune 21, 1966 R. l.. TANNER 3,257,661

LOG-PERIODI() ANTENNA Filed April ll, 1962 5 Sheets-Sheet 4 F 22a 22b222d 15- 22e 120 ,f 229 /22h S 12! s" 122/ f/ 123 22f w w s* s v v' /N/N/N/NN/N/N/MMWMM@ INVENTOR ROBERT L. TAN NER 22a ATTORNEY June 2l, 1966 R. L. TANNER 3,257,661

LOG-PERIODICI ANTENNA Filed April ll, 1962 5 Sheets-Sheet 5 15.2 1| Flg- 5 Z lll/l0 Fig/ i5 b INVENTOR.

ROBERT L. TANNER ATTORNEY United States Patent O 3,257,661 LOG-PERIODIC ANTENNA Robert L. Tanner, 4780 Alpine Road, Menlo Park, Calif. Filed Apr. 1,1, 1962, Ser. No. 186,734

22 Claims. (Cl. 343-7925) This invention relates to log-periodic antennas, and more particularly to a log-periodic antenna of narrow beam width and great bandwidth.

Basically, log-periodic antennas are antennas cornprised of an array of elements in which each element is dimensioned and positioned such that the electrical properties repeat periodically with the logarithm of the frequency. Relative frequency independence is obtained when the variation of theelectrical property is small and therefore the period is small.

A well-known log-periodic antenna is the log-periodic dipole array in which the individual elements comprise dipoles dimensioned and positioned relative to one another so lthat both the length and the spacing change in a log-periodic manner. In other words, the spacing between adjacent elements and the respectivelength of adjacent elements changes by a ixed percentage from one element to the next. The dipoles are excited in the one-half wavelength mode by a uniform two-wire feed conductor with the wires transposed between adjacent dipoles. See generally Antenna Engineering Handbook by Henry lasik, Editor, McGraw-Hill Book Company, Inc., Chapter 18, sections 18.3 and 18.4, 1961.

As fully described in the above referred to Handbook, the log-periodic dipole array utilizes feed conductor transpositiwons to obtain radiation in the direction of the small end of the array, i.e., back along the direction of the feed conductors. one array element is excited at any one time, the transposition of feed conductors provides the necessary 180 phase difference between the currents excited in adjacent array elements so that their combination results in an electric eld radiated in the backward direction.

Beam widths corresponding to directive gains of the order of about l() db are obtainable with the log-periodic dipole array. For many applications it would be very desirable to have directivities substantially higher than ythis while at the same time retaining the frequency independence of pattern and impedance which characterize the log-periodic anntenna. Since the log-periodic dipole array comprises a structure of periodically repeating electrical properties the bandwidth is determined by the number of elements of which the antenna is comprised, the rst and last element providing the upper and lower frequency limits.

To overcome the limitations on directivity experienced in the log-periodic dipole array, `a log-periodic resonant-V array has been developed, in which the individual elements are arranged to form a V with the apex opposite the direction of radiation. This antenna may be operated in modes higher than the one-half wavelength mode characterizing the normal log-periodic dipole array. The log-periodic resonant-V array is fully described in a paper entitled Logarithmically Periodic Resonant-V Array by Mayes and Carrel, published at the 1961 Western Electronic Show and Convention, San Francisco, August 1961 as Paper 1/1.

Operation of the array elements in the higher modes makes it possible to utilize electrically larger elements having higher individual gain, and consequently to achieve substantially higher gain for the array as a whole. Elements of the resonant-V array may have a length of three-half wavelength, five-half wavelength, etc. Arranging the individual elements in the form of a V is necessary to achieve unidirectional radiation patterns for them More particularly, since more than 3,257,661 Patented June 2l, 1966 ice when :operated in a higher order mode. By this technique the directive gain of the antenna can be increased, for example, to 12 db in the three-half wavelength mode and to 17 db in the seven-half wavelength mode. However, since the array is constructed for operation in a selected mode, its broadband characteristics are limited by that mode.

For example, if the array is constructed for operation in the three-half wavelength mode, the shortest element (that is the combined length of both sides of the V) is dimensioned -to be equal to three-halves of the shortest operating wavelength, say 50 centimeters, for a 33.3 cm. operating wavelength. As this operating wavelength increases (or the frequency decreases) the next larger elements are excited until the operating wavelength be- 'comes 10() cm., at which point the shortest element will again be excited but this ltime in the one-half wavelength mode, the one-half Wavelength resonance of the shorter elements will effectively prevent energy at frequencies below that at which the wavelength is cm.l

(]i=300 mc.) from reaching Ithe longer elements to excite them in the three-half wavelength mode. In addition, the simultaneous presence of two modes on the antenna, for the very narrow range of frequencies (near 300 rnc. in this case), where it is possible to excite them, disturbs both the pattern and impedance characteristics of the antenna as explained by Mayes and Carrel. Consequently, it is useless to make elements longer than .approximately crn. or three times the length of the shortest element.

From the above it is seen that for operation in the three-half wavelength mode the operating frequency is limited to a 3:1 range.

More generally, if the antenna is to be operated in the n-half wavelength mode, the frequency limits are approximately those at which the smallest element is in length to (n2)-half wavelength on the low frequency end of the range and equal to n-half wavelengths on the high frequency end of the range. The bandwidth is therefore n:(n-2). Accordingly, even though beam sharpness has been increased, the severely restricted bandwidth of the log-periodic resonant-V array is a limitation making it unsuitable for many applications.

It is therefore a primary object of this invention to provide an improved log-periodic antenna having a bandwidth which is essentially unlimited as in the case of the log-periodic dipole array, having a beam sharpness equal to or greater than that of the log-periodic resonant-V array, and having a physical size small relative to the directivity obtained.

It is another object of -this invention to provide a logperiodic antenna in which the elements are excited with higher order modes and in which the bandwidth and the beam sharpness is greater than has been possible heretofore.

It is still another object of this invention to provide a log-periodic antenna which can be operated over a substantially greater frequency range lthan the log-periodic resonant-V array and which provides a beam having a substantially greater sharpness than the logperiodic dipole array.

It is a further `object lof this antenna to provide an efficient and versatile log-periodic `antenna which has good bandwidth and directivity properties and low sidelobes and back-lobes.

It is a still further object of this invention to provide a log-periodic antenna in which the strength and phase of the excitation of different parts of the antenna can be adjusted in -such a way as to achieve a maximum of directivity and a minimum of .sideand back-lobe radiation.

It is a still further 'object `of `this. invention to provide a ilog-periodic antenna including me-ans 'for 'substantially restricting the mode to be excited to a single selected mode or combination ot modes.

Briefly, the log-peri-odic antenna of this invention comprises a log-periodic array ot elements similar to the log-periodic resonant-V antenna, in which the individual elements are excited by one or more feed conductors, at least one `feed conductor being connected to each element at a point intermediate between the ends of the element of ithe unbalanced (one-half) structure. Phase shift between adjacent active elements is provided by the lfeed lconductors and 4is maintained to exceed 180 so that Ithe current in one active array element effectively [leads in phase the current of the next smaller active element. Also, in case more than one feed conductor is utilized tor each unbalanced structure, adjacent conductors are so arranged as `to provide a relative phase shift therebetween at the appropriate elements yof substantially 180 `so in -the appropriate high `order of shorter elements rin `lower `order modes is minimal, or alternatively, is controlled so lthat radiation Ifrom the lower order modes augments the radiation from the high order modes. Further, the phase shifts between adjacent array elements and adjacent yfeed conductors are obtained by a slow wave 'lfeed caused by series impedance and shunt admittance loading with the individual array elements.

Other yobjects and a better understanding of the inventionmay be had -by reference to fthe following description, taken in 'conjunction with the accompanying drawings, in which:

FIG. 1 is half of a ylog-periodic -antenna erected over a ground plane in which the array elements of the nnbalanced structure are excitable for operation in the onehalf Wavelength mode by a radial lfeed conductor;

FIG. 2 is a vector diagram useful in explaining the operation of `this invention;

FlIG. 3 is a ifulil log-periodic antenna constructed o f a pair of half antennas as I'shown in FIG. 1 to form a balanced structure;

FIG. 4 is half of a log-periodic antenna erected over a ground plane in which the array elements ofthe unbalanced structure are excita-ble for operation in the onehalf wavelength =mode by a staggered deed conductor;

FIG. 5 is a full log-periodic antenna :constructed of a pair o-f half antennas as shown in FIG. 4 to torm a balanced structure; IFIG. 6 is halff ofa log-periodic a ground plane in which the yarray balanced :structure are excitable wavelength mode;

FIG. 7 is half of a log-periodic antenna erected over a ground plane in Which the array elements of the unbalanced structure are excitable for operation in the three-half wavelength inode;

LFIG. l8 is halt of a log-periodic antenna erected over aground .plane in which 4the -array elements of the unbalanced structure are excitable for operation in the threequarter wavelength mode;

FIG. 9 is half lof a ylog-periodic antenna erected over a grou-nd plane in which the array elements of the unbalanced structures are jive-quarter wavelength mode;

PIG. 10 is half of a log-periodic antenna erected over a Aground plane in 'which the array elements of each of the unb-alanced structures are excitable Ifor operation in the yone-half wavelength mode by a pair of transposed feed conductors;

IFIG. 11 shows a full log-periodic yantenna constructed of a pair of half `antennas as shown in FIG. l0;

FIG. 12 :is a ttull log-periodic antenna in which the array elements of the balanced structure are excitable modes, while .the excitation `antenna erected over elements of the vunifor operation in the one `that these elements are strongly excited excitable for operation in the' for operation in the three-half wavelength mode by three pairs of transposed feed conductors; and

\FIGS. 13a and 13b are yrespectively a top view and an elevational view of a dihedral log-periodic antenna constructed in accordance with this invention.

Referring now to the drawings, and particularly to FIG. 1 thereof, there is shown a log-periodic antenna 29 constructed in accordance with this invention comprising a plurality of array elements 22a, 22h, 22C, 22d, 22e the ylargest element always carrying the lsubscript a. Antenna 20 is erected over a g-round plane 21 and is therefore referred to as half of a full log-periodic antenna. As is well lknown to those skilled in the art, erecting an antenna over a ground plane is to provide a counterpo-ise which images the structure abo-ve the ground. Antenna 20 is excited by a signal source 24 which applies its signal to the individual array elements 22a, 22h, 22C viaa iced conductor 25.

Array elements 2|2a, 1221i, 22C antenna 20 are collectively referred to .simply by reference character 22. Arr-ay elements 22 diier from one another in length and adjacent spacing by a xed percentage. For example, if array element 22C is selected to Ibe 30.0 cm. long and the xed percentage is selected as l10%, element `=22b has a length of 33.0 cm. and element 22a has a length of 36.6 cm. Similarly, if vthe spacing between element 22C and `22d is `selected to be 5.0

of ylog-periodic "cm, the distance between 22e and "22b is 5.5 cm. and the distance between elements 22a and 22b is 6.1 cm.

Since the antennas shown in Ithe subsequent gures are likewise log-periodic antennas constructed in logperiodic fashion, the-same yreference character will be employed in referring to .the individual array elements thereof. it is, of course, understood that the percentage ratio of change of :the lengthvand -the spacing of the individual array element is selected to suit the particular mode of excitation. likewise, the signal Isource for ex- -citing the several antennas shown in the subsequent iigures will be given :the same reference character, being understood that different 'frequency ranges are used with the different antennas depending on the mode to vbe excited and the length of the array elements.

The individual array elements 22 are inclined with respect to the array axis 26, the angle of inclination being Iselected for maximum gain. It has Ebeenvfound that the directivity of a ibcam 27 varies to some extent with the angle lof .inclination for different modes. For example, for the one-half wavelength mode of operation of the full, or balanced, structure suitably excited, the angle between elements 22 and a plane perpendicular to array axis for a maximum gain is zero. Such an angle, of course, reduces the antenna to a conventional log-periodic dipole array. For maximum directivity the angle was found to be about 32 degrees for operation in the three-half wavelength mode, 50 degrees for operation in the five-half wavelength Imode, and 52.2 degrees for :the seven-half wavelength mode for a complete (not half) array element. As described in the above referred to Paper 1/1, an experimental study showed that this angle is not very critical and a change of l0 degrees or more has a rather small effect on the main-lobe level, its principal eiTect being a change of side-lobe level.

The term log-periodic resonant-V array, as used in here, deiines a plurality of array elements arranged and dimensioned in a l-og periodic Imanner to form a log-periodic array and inclined with respect to the axis of radiation.

Returning now lspecifically to the description of FIG. 1, lfeed conductor 25 has the form of a straight wire and is connected .to the center of each array element 22 of the unbalanced structure to excite elements 22 in the one-half wavelength mode 28. Since individual elements 22 are different lengths it is readily seen that the array element most strongly excited will be the element Whose length is nearest to a resonant value, which, in practice, is close to one-half Wavelength of the operating frequency. Say that this is element 2.2b. Element 22h therefore contributes most strongly to the radiated energy. Although maximum 'current is flowing on ele-ment 2211, it is readily understood that array elements 22a and 22C, adjacent to resonating element 221), `are also near resonance and will there- Ifore be excited also, though less strongly than 221). rFhey Will also contribute, therefore, to the total energy radiated.

Feed conductor has a number of important properties and characteristics 'by virtue of being crossed and connected to array elements 22. The effe-ct of crossed wires connected to a transmission line is to provide loading which slows down the wave substantially. This loading, for purposes of analysis, may lbe considered to be of two types.

As fully explained in a paper entitled Wave Propagating Properties of a Pair of Wire Grids with Square, Hexagonal and Triangular Mesh, by R. L. Tanner and M. G. Andersen (to lbe published in the Transactions of the IRE Professional Group `on Antennas and Propagation, 1962), when the cross conductors are orthogonal to the feed conductor they introduce a periodic shunt loading admittance. Depending on whether the cross wires are shorter or longer than the resonant length, this shunt loading admittance will be capacitive or inductive. At resonance it will have a high value and will appear as a conductance, representing radiation of energy from the cross Wire.

When the cross wires make other .than a right angle with the feed conductor, as in FIG. l, or when they have an element of length in common with the feed conductor as in FIG. 4, it Will 'be evident to those skilled in the art that there is a mutual inductance between the feed conductor and the cross wire which causes a current flowing on :the cross Wire to induce a voltage in series with the feed conductor. This series voltage can be represented lfor analysis purposes by an impedance in series with the feed conductor. When the vcross wire is shorter than the resonant length this equivalent series impedance is essentially an inductive reactance When it is longer than a resonant length the series impedance is capacitive. For a cross wire of resonant length the series impedance is a resistance representing radiation and has a high value.

Thus, the cross Wires provide both series impedance loading and shunt admittance loading. The shunt admittances can be represented approximately by'series resonant circuits having a high value of admittance at re-sonance, while the series impedances can be represented approximately by parallel resonant circuits having a high value of impedance at resonance.

It is important t-o note that the magnitude of the series impedance can be altered `and adjusted by changing the angle `between the `feed conductor, `or by increasing the common length of feed conductor and cross wire. In `antenna 20, elements 22 cross feed conductor 25 at an angle so that both series and shunt loading are present, which loading is carefully selected to provide a phase shift between -adjacent array elements which exceeds 180 degrees for the Agroup of elements having lengths near thc resonant length.

The operation of antenna 20 may be 'best be explained with the aid of `the vector diagram of FIG. 2 which substantially, even though not entirely accurately, shows the manner of operation. Let it be assumed that antenna 20 is excited by generator 24 with a frequency corresponding :to the resonant frequency of element 22h. Accordingly, element 22b is most strongly excited and becomes the 4major contributor to the total energy radiated. Let the current excited in element 22b be Ib and the electric field generated by current Ib be Eb. Since the electric field Eb is radiated, its phase varies with position so that Eb at element 22h is indicated as (Eb)b. In FIG. 2 the current Ib is shown by line 31 and the field (Ehh, is shown by line 32.

As has already lbeen mentioned, since elements 22a and 22C are also near resonance, both of these elements Will 'be excited even though to a lesser degree than for 22h. This -is 4also true of the other elements to a still lesser degree but their contribution will be neglected herein. Let the currents in element 22a and 22C be Ia and Ic respectively, and the electric fields due t0 current I,l and Il, be (Baja and (Eck.

As has been explained heretofore feed conductor 25 is constructed as a loaded transmission line `which slows down the wav-e by more than 180 degrees bet-Ween adjacent array elements so that the current in the larger array element effectively leads the current in the adjacent smaller -array element.

Current Ia may therefore be shown as line 33 and field (Ea)a as line 34 and are obtained by rotating line 31 in a clockwise direction equal to the phase shift between element 22h and 22a. As shown in FIG. 2 :this phase shift is approximately equal to 235. Similarly, current lc shown as line 3.5 `and the field (EQ,3 shown as line 36 are obtained `by rotating line 31 counter-clockwise through an angle equal t-o the phase shift along feed conductor 25 between element 22C and 22h.

Toobtain the combined effect of the several electric fields, a point X may be selected along the direction of radiation and the electric fields due to elements 22a, 22b, and 22C translated to the point X. The elds due to each individual array element travel towards point X with the speed of light, traversing different distances and therefore undergoing different phase shifts.

The phase change suffered by electric field Ewin going from a to x, is indicated by angle 37 and the electric field (E)x is indicated by line 38. Similarly, electric eld Eb, in going from b to x, suffers a phase change indicated by angle 39 and (Eb)x is shown by line 40. Angle 39, of course, is less than angle 37 by an amount equal to the phase change suffered in free-space in moving a distance equal to the separation between elements 22a and 22h. Similarly E., at c shown at 36 suffers a small phase change indicated by angle 41 in going from element 22C to x and electric field (Ec)X is shown by line 42. Accordingly, the electric fields at X due to element 22a, 22b, and 22C are shown by vectors 38, 40 and 42.

The total field at point X is obtained by vectorially adding (E)|(Eb)|(Ec)X to obtain the rresultant shown by line 43. In order for vector 43 to b'e a maximum it is necessary to carefully provide for phase shift between adjacent elements such that the resultant vectors at a point such as X due to the several elements are as much in phase as possible. This is accomplished if the phase shift between adjacent active elements is substantially greater than It is for this reason that feed conductor 25 is loaded to provide a selected phase shift which may at times be as large as or larger than 300, depending on the proximity of adjacent elements and other parameters of the antenna.

Referring noW to FIG. 3 there is shown a full logperiodic antenna 50 of the balanced typ'e in which array elements 22a, 22h, 22C are excitable in the full Wavelength mode 51. Antenna 50 essentially is constructed of two half sections, each of which is substantial- `ly like antenna 20 of FIG. l and fed with a two-wire feed conductor 52 connected across the output terminals of generator 24. Operation of antenna 50 is substantially the same as antenna 20 except that the radiation pattern is symmetric about the array axis.

FIG. 4 is a half of a log-periodic antenna 60 constructed over a ground plane 21 very much like the antenna 20 of FIG. 1, having array elements 22 excitable in the one-half Wavelength mode 69 of the unbalanced structure. The distinction between antenna 60 and antenna 20 lies in the particular feed conductor utilized for exciting the individual elements 22. In antennas 20 and 50, feed conductors 25 and 51 comprise a radially extending straight conductor. Antenna 60, on the other hand, is excited by a staggered feed structure including conductors 61, 62, 63 and 64, each conductor connecting a pair of adjacent array elements. In this manner, current from generator 24 flows from element 22e to element 22h sequentially along conductor 64, along element 22d, along conductor 63, along element 22C, and thereafter along conductor 62 to element 22h.

As has been explained hereinabove, loading of the feed -conductor to provide the desired phase shift betwen adjacent elements is partially by an effective shunt admittance and partially by an effective series impedance. Since the degree of shunt loading is substantially constant, a change of the series loading is one convenient parameter to change the phase shift.

Staggering of feed conductors to provide a common path with an element such as the common path provided by element 22d between feed conductors 64 and 63 results in maximum mutual coupling when element 22a' is excited. As explained hereinabove, this coupling provides a variable mutual inductance by which the magnitude of the series impedance can be altered, and results in a phase shift, the amount of phase shift being controllable by the length of common path between conductor 64 and 63. In other words, staggering feed conductors provides a convenient and additional parameter for -adjusting the phase shift to coincide with the desired phase shift for maximum beam sharpness.

FIG. shows a full log-periodic antenna 70 constructed of a pai-r of half antennas such as shown in FIG. 4 to provide a balanced structure. In fact, antenna 70 is very much like antenna 50 of FIG. 3 except that staggered feed conductors 71 are provided for increasing the phase shift between adjacent array elements 22 by maximizing the series impedance.

FIG. 6 shows a half log-periodic antenna 80 erected over ground plane 21 and fed by generator 24. Antenna 80 comprises array elements 22 which are of such length that they may be excited for operation in the full wavelength mode S5 of the unbalanced structure at the operating frequency. To excite the full wavelength mode properly a pair of feed conductors generally indicated by reference characters 81 and 82 are utilized. Feed conductors `81 and 82 are respectively connected to `each array element 22 at the one-quarter and three-quarter point along the direction of extension. I

In the preferred embodiment the two feed conductors 81 and 82 shown are of the staggered variety so that proper loading may be provided. However, it is within the contemplation of this invention to utilize radial feed conductors such as are shown in FIG. l, the stagge-red feed conductors merely providing the advantage of an additional parameter for adjusting the phase shift between adjacent elements for maximum beam sharpness and a phase shift between adjacent feed conductors of 180.

As will become immediately apparent from a vector diagram similar to the one shown in FIG. 2, it is necessary that the current applied to a particular array element of the unbalanced structure by feed conductorsV 81 and 82 be 180 degrees out of phase. The reason for this, of course, is obvious since the individual array elements make an angle with the array axis the radiation from the upper portion being considerably closer to a point on the beam axis than the radiation from the lower portion of the element. A vector diagram just like the one shown in FIG. 2 can easily be constructed to show that maximum beam sharpness is obtained by feeding the upper and the lower portion of each element 180 out of phase.

Of course if one of `feed conductors 81 or `82 were removed, the same full wavelength mode 85 would be excited. However, if the -operating wavelength is doubled the same element would be excited in the one-half wavelength mode and would prevent energy from reaching the element which is of proper length to excite the full wavelength mode. Utilizing a second feed conductor reduces the excitation of lower order modes, enabling energy to reach the portion of the antenna which will be excited in the high order mode, thereby increasing both the bandwidth and gain of the antenna. Accordingly, a properly positioned separate feed conductor for individually exciting every one-half wavelength portion of the desired high order mode provides a means for controlling the excitation of lower order modes, enabling them to be eliminated or, more desirably, tobe excited in such a manner as to augment the radiation of the high order mode, thereby increasing antenna gain.

As a consequence of utilizing separate feed conductors for each one-half wavelength' portion of the high order mode elements 22 may be broken at their mid-points (zero current) and connected by insulators 83. This technique eliminates completely the low order modes, which may under some circumstances be desirable.

FIG. 7 shows a half of a log-periodic antenna 90 constructed over a ground plane 21 and utilizing three feed conductors 91, 92 and 93 for exciting the array elements 22 in the three-half wavelength mode 94. Generator 24 is connected between ground and feed conductors 91, 92 and 93.

Elements 22 are dimensioned in length for operating at the three-half wavelength mode at the operating frequency. Again, .as in the case of antenna 80, elements 22 may or may not be broken by insulators at points midway between the feed conductors. The same reasoning given in connection with the operation of antenna of FIG. 6 applies here also, namely that the currents applied to a single active (near resonant) array element by the feed conductors must be 180 degrees out of phase. Also, as in the case of antenna 80, both low order and high order modes can be excited, but excitation of the low order modes can be controlled so that their radiation augments the radiation of the high order modes. Of course, antenna may also be excited properly by radial feed conductors as shown in FIG. 1.

FIG. 8 shows a half of a log-periodic antenna 100 constructed over a ground plane 21 in which the individual array elements 2'2 are dimensioned for operation in the three-quarter wavelength Inode 103 at the operating frequency. To provide the proper excitation, staggered feed conductors 101 and 102 are utilized and connected to one terminal of a generator 24, the other side lof which is connected to ground plane 21. Feed conductors 101 and 102, shown in staggered form (even though radial feed conductors may be utilized), a-re connected respectively to the ends of array elements 22 nearest the array axis and to the two-thirds point.

Here feed conductor 101 excites the one-half wavelength portion and feedconductor 102 excites the onequa'rter wavelength portion of mode 103. Consequently, array elements 22 may be broken at the one-third point where the current is zero. In this manner, as can easily be seen, the three-quarter mode 103 is excited by feed conductors 101 and 102 properly loaded to provide substantially a degree phase shift between adjacent feed points on the active `(near resonant) elements.

FIG. 9 shows a half of a log-periodic antenna 110 erected over a ground plane 21 in which the individual array elements .22 are excitable, at resonance, in the fivequarter wavelength mode 114 of the unbalanced structure. To excite the five-quarter wavelength mode 114 in elements 22 at resonance, threefeed conductors 111, 112 land 113 are utilized and respectively connected to the inner end portion, the two-fths point and four-fifths point along the length of each element 22. Phase shifts along the individual conductors 111, 112 and 113 are so adjusted that along each active element the energy from generator 24 is 180 shifted with respect to adjacent feed conductors.

More particularly, feed conductors 112 and 113 each excite a separate one-half wavelength portion of the five-quarter wavelength mode 114, the remaining one-` quarter wavelength portion being excited by feed conductor 111, The relationship between the number of feed conductors and their position with respect to the mode to be excited may be generalized as follows: For a log-periodic resonant-V antenna erected over a ground plane, the number of feed conductors 11, in accordance with this invention, is equal to the total number of onehalf wavelength portions of the highest order mode to be excited plus one for the odd one-quarter wavelength portion of said mode-if there be such. Adjacent feed conductors are separated by one-half of the resonant wavelength of the element and are alternately 180 degrees out of phase. For example, an antenna constructed over a ground plane for operation in the nine-half wavelength mode requires nine feed conductors and for operation in the thirteen-quarter wavelength mode requires seven feed conductors.

The antennas of FIGS. 6, 7, 8 and 9 are merely exemplary of a large class of log-periodic arrays excitable not only in the mode shown but excitable in higher order modes by utilizing additional feed conductors. For example, a two-full wavelength mode antenna may be constructed similar to the antenna shown in FIG. 6 by utilizing four radial or staggered feed conductors. A five-half and seven-half wavelength mode antenna may be constructed similar to the antenna shown in FIG. 7 by `utilizing respectively four and tive adjacent staggered or radial feed conductors. A seven-quarter and ninequarter wavelength mode antenna may be constructed similar to the antenna shown in FIG. 8 or FIG. 9 by respectively utilizing 4 and 5 adjacent staggered or radial conductors. Furthermore, each of the half antennas shown may be converted into a balanced full log-periodic antenna by replacing the ground plane with another half antenna as will be immediately obvious to those skilled in the art.

Referring now to FIG. l0, there is shown a structure in the form of half of a log-periodic antenna 120 constructed over a ground plane 21 and excitable for operation in the one-half wavelength mode 123 of the half element 22 shown. Antenna 120 may be looked at in two different ways. One Way is that antenna 126 comprises a single half of a log-periodic'antenna having a period equal to one-half ofthe period of antenna 20 of FIG. 1 in which the array elements 22 are alternately connected to feed conductors 121 and 122 across which a balanced generator 24 is connected. In this manner, antenna 120 may be regarded as having array elements 22 excited by a transposed (alternately connected) feed conductor so that a 180 degreephase shift is built directly into the feed structure. This feed conductor transposition may, of course, be used with any of the previously described antennas providing the loading of the feed conductor is less than 180 degrees so that proper relative excitation is provided.

Another way of looking at antenna 120 is to regard array elements 22a, 22C, 22e, 22g as comprising one antenna and array elements 22b, 22d, 22f as comprising another antenna. Both antennas have the same log-period but the largest array element of one antenna differs from the largest array element of the other antenna by one-half of the log-period. These two antennas are then physically interleaved and excited 180 degrees out of phase by connecting their respective feed conductors to the plus and minus side of generator 24, or, alternatively, the feed conductor of one antenna of the inter-leaved structure can be connected to ground aand the other antenna fed by an unbalanced generator.

As a practical matter, either explanation of the makeup of lantenna 120 provides the same result, namely that adjacent array elements have transposed feeds. Antenna 120 may be utilized with radial or staggered feed conductors, or a combination thereof. Individual staggering of the feeds provides an additional degree of control and permits optimization of the antenna pattern and impedance.

FIG. 11 shows a full log-periodic antenna 130 con- 1t) structed of a pair of half log-periodic antennas such as shown in FIG. 10 eliminating ground plane 21.

Referring now to FIG. 12 there is shown a full logperiodic antenna in which each array element 22 is dimensioned for excitation in the three-half wavelength mode 147. Each array element 22 of the full antenna is fed by four out of six feed conductors 141, 142, 143, 144, and 146 each of which is transposed (alternated) between adjacent array elements 22. For example, along the array axis of antenna 140 the adjacent ends of element 22 are fed by transposed feed conductors 141 and 142. The upper elements 22 are connected alternately to feed conductors 145 and 146 and the lower elements 22 are connected alternately to feed conductors 143 and 144. In this manner a phase shift of 180 degrees is directly built into the feed structure of antenna 140 by transposition of feeds. Additional phase shift is provided by feed conductor loading due to crossed wires as has already been explained.

FIGS. 13a and 13b show an embodiment of this invention particularly useful for long-distance vcommunication. A log-periodic antenna 156 is formed of two half log-periodic antennas 151 `and 152 which are alike in all respects and fed by the same signal source (not shown). Antennas 151 and 152 are arranged side by side in a dihedral manner, each antenna defining one of the two planes. Because of the dihedral configuration, antenna may be referred to as a dihedral antenna and an; tennas 151 and 152 as its component antennas.

Component antennas 151 and 152 may be selected to have any of the forms described hereinabove and if excitation in the full-wave wavelength mode of each component antenna is desired, antennas 8l) of FIG. 6 may be utilized. Dihedral antenna 150 is erected over the ground plane 21 and spaced therefrom by a plurality of support elements such as wooden poles 153. The fold line 154, defined as the apex of the dihedral angle between component antennas 151 and 152, is inclined with respect to ground plane 21.

Both the dihedral angle (angle between component antennas 151 and 152) and the inclination of fold line 154 with respect to ground plane 21 are selected to provide a beam 155 (far-zone pattern) of a desired width having a beam axis 156 properly directed for ionospherical reflection to a chosen distant point.

Except for arranging the two component antennas 151 and 152 to for-m a dihedral plane and erecting the resulting structure over ground plane 21, antenna 150 is very similar to a full log-periodic antenna, such as antenna 5t) or 70 (operated in the one wavelength mode of the full array) or antenna 140 (operated in the three-half wavelength mode of the full array) folded along the array axis.

There has been described hereinabove a whole new class of log-periodic `antennas having a bandwidth which is determined by the electrical length and the selected.

operational mode of excitation of the smallest and largest array element in the array and a beam sharpness determined by the selected operational mode of excitation.

VAll elements are excited in a selected higher order mode than the one-half wavelength rnode (quarter wavelength mode for an unbalanced antenna constructed over a ground plane) by feed conductors which provide a phase shift greater than between adjacent array elements A feed conductor is provided for every one-half wavelength portion of the mode to be excited (and an extra one in case there is an additional odd onequarter wavelength portion) to suppress the generation of lower order modes. In this manner the bandwidth of the antenna of `this invention may be made as large as desired. If more than one feed conductor is connected to a single element of an unbalanced structure an addi tional 180 phase shift is provi-ded between adjacent feed points.

What is claimed is:

1. A log-periodic antenna comprising: a plurality of array elements arranged and dimensioned to form a logperiodic array of elements inclined with respect to an array axis; each of said array elements comprising an elongated radiating conductor; and feed means coupled to each of said elements for restricting excitation of said array to a selected combination of modes, said feed means including a feed conductor coupled to an elongated radiating conductor at a location between its ends determined by the radiation mode, said feed conductor making an angle with said elongated radiating conductor to produce a predetermined phase shift.

2. A log-periodic antenna comprising: a plurality of array elements arranged and dimensioned to form a logperiodic array of elements -of the resonant-V type; and feed means coupled to said array elements for restricting excitation 4of said array to a selected combination of modes, said feed means including a different feed con ductor for separately exciting each one-half wavelength portion of the highest order of said selected combination of modes.

3. A log-periodic antenna comprising: Ia plurality of oppositely inclined array elements arranged in pairs and dimensioned to form a log-periodic array of elements;

,a pair of feed conductors respectively connected along their lengths to mid-points of each pair of array elements; and frequency generator means, said pair of fee-d conductors being coupled to said generator means for exciting the one wavelength model of the pair of array elements which are near resonance.

4. A log-periodic antenna comprising: a plurality of array elements arranged and dimensioned to form a logperiodic array of elements of the resonant-V type; and feed means coupled to said array elements for restricting excitation of said array to a selected combination of modes, said feed means including a separate feed conductor for separately exciting each one-half wavelength portion of the selected highest order mode, and a further feed conductor for exciting the one-quarter wavelength portion if there be one, of said selected highest order mode.

5. A log-periodic antenna excitable in a selected combination of modes, each mode of said combination having an integral number of one-half wavelength portions, said antenna comprising: a plurality of array elements arranged and dimensioned to form a log-periodic .array of inclined elements above a ground plane; and feed means coupled to each of said elements, said feed means including a separate feed conductor for exciting a different one-half wavelength portion of the highest order mode of said selected combination of modes at resonance of an array element.

6. A log-periodic antenna excitable in a selected combination of modes, each mode of said combination having an odd integral number of one-quarter wavelength portions, said antenna comprising: a plurality of array elements arranged and dimensioned to form a log-periodic varray of inclined velements above a ground plane; and

feed means coupled to each of said elements, said feed .means including a separate feed conductor for exciting a different one-half wavelength portion of the highest order mode of said combination of modes and a further feed conductor for exciting the odd one-quarter wavelength portion directly opposite said ground plane of said mode at resonance of an array element.

7. A log-periodic antenna comprising: a plurality of array elements arranged and dimensioned to form a logperiodic array of outwardly inclined elements on opposite sides Vof an array axis; a signal source having a pair of out-of-phase output terminals; and feed means for restricting excitation of said array to a selected mode connecting said pair of output terminals respectively to said array elements on opposite sides of said array axis, each said feed means comprising a separate feed conductor for every one-half wavelength portion of said selected mode, said feed conductors being connected to said elements at a point one-quarter wavelength of the resonant wavelength from its outer end portion and thereafter successively at points one-half wavelength of the resonant wavelength from the previous coupling point, the most centrally positioned feed conductor being no farther than one-quarter of the resonant wavelength of the element from the element portion closest to said array axis.

8. A log-periodic antenna in accordance with claim 5 in which adjacent feed conductors along any given element at resonance excite the resonating element with out-ofphase currents.

9. A log-periodic antenna in accordance with claim 6 in Which adjacent resonating array elements are excited more than degrees out-of-phase with one another.

10. A log-periodic-antenna in accordance with claim 6 in which said feed conductors are straight line conductors inter-connecting successive array elements to provide sufiicient loading upon said feed conductor to produce a phase shift greater than 180 degrees when said elements are excited in said selected mode.

11. A log-periodic antenna in accordance with claim 6 in which said feed conductors are stepped so that the array elements form a portion of said feed conductors.

12. A log-periodic antenna comprising: a plurality of array elements arranged and dimensioned to form a logperiodic array of inclined elements; and feed means coupled to said elements for exciting said antenna in a selected mode, said feed means including a plurality of feed conductors -coupled to progressively larger array elements and being spaced from one another along any one element a distance equal to one-half of the resonant wavelength of said element for said selected mode, adjacent feed conductors providing substantially a 180 degree outof-phase excitation to the resonant element, and a greater than 180 degree out-of-phase excitation between elements adjacent to the resonating element.

13. A log-periodic antenna excitable'in a selected mode comprising: a first and a second plurality of array elements arranged and dimensioned to form a first and a second log-periodic array of elements on opposite sides of an array axis, the elements of said first and second array being arranged in pairs inclined to form a V open towards the direction of radiation; a signal means having first and second terminal means; and irst and second feed means connecting said irst and second terminals to rst and second array respectively, each of said feed means including a feed conductor for each one-half wavelength portion of said selected mode for each array which is connected to each element of the array, and a further feed conductor connected to the inner end of the associated array if the selected mode includes an odd number of one-quarter wavelengths.

14. A log-periodic antenna constructed over a ground plane and restricted to excitation in the one-half Wavelength mode of the unbalanced structure, said antenna comprising: a plurality of array elements arranged and dimensioned to form a log-periodic array of elements outwardly inclined in the direction of radiation; a frequency generator means; and a feed conductor connecting said generator means to the midpoint of successively larger array elements, said feed conductor being arranged and interconnected with said array elements to provide suicient loading to correspond to a phase shift greater than 180 degrees between a resonating array element and array elements immediately adjacent said resonating array element.

15. A log-periodic antenna constructed over a ground plane and restricted to excitation in the one wavelength mode of the unbalanced structure, said antenna comprising: a plurality of array elements arranged and dif mensioned to form a log-periodic array of elements outwardly inlined in the direction of radiation; a frequency generator means; and a first feed conductor connecting said generator means to the one-quarter point and a second feed conductor connecting said generator means to the three-quarter point of successively larger array elements, said feed conductors being arranged and interconnected with said array elements to provide suicient loading to correspond to a phase shift greater than 180 degrees between a resonating array clement and array elements immediately adjacent said resonating array element and to correspond to a phase shift of substantially to 180 degrees between said one-quarter and said threequarter point of said resonating element.

16. A log-periodic antenna constructed over a ground plane and restricted to excitation in the three-half wavelength mode of the unbalanced structure, said antenna comprising: a plurality of array elements arranged and dimensioned to form a log-periodic array of elements outwardly inclined in the direction of radiation; a frequency generator means; and a first feed conductor connecting said generator means to the one-sixth point, a second feed conductor connecting said generator means to the one-half point and a third feed conductor for connecting said generator means to the five-sixth point of successively larger array elements, said feed conductors being arranged and interconnected with said array elements to provide sufficient loadingto correspond to a phase shift greater than 180 degrees between a resonating array element and array elements immediately adjacent with resonating array element and to correspond to a phase shift of substantially 180 degrees between said one-sixth and said one-half point and between said onehalf and said live-sixth point of said resonating array element.

L17. A log-periodic yantenna constructed over a ground plane 4and restricted to excitation in the three-quarter wavelengtlnmode of the unbalanced structure, said antenna comprising: a plurality of array ele-ments arranged and dimensioned to fonm a log-periodic array of elements outwardly inclined in the direction of radiation; a frequencyA generator means; and a first feed conductor connecting said generator means to the end point and a second -feed conductor connecting said generator means to the two-thirds point of successively larger array elements, said feed conduct-ors being arranged and interconnected with said array elements to provide sufficient loading to correspond to a phase shift greater than 180 degrees between a resonating array element and array elements immediately adjacent said resonating `array element and to correspond to ya phase shift of substantially 180 degrees between said end point and said two-thirds point o-f said resonating array element.

I18. A log-periodic antenna constructed over a ground plane and restricted to excitation in the dive-quarter wavelength mode of the unbalanced structure, said antenna comprising: a plurality of array elements arranged and dimensioned to form a log-periodic array of elements outwardly inclined in the direction of radiation; a frequency generator means; and a first feed conductor connecting said generator means to the end point, a second feed conductor connecting said generator means to the two-fifths point and a third tfeed conductor `for connecting said generator means to the four-fifths point of successively larger array elements, said feed conductors being arranged and interconnected with said array elements to provide suflicient loading to correspond to a phase shift greater than 180 degrees between a resonating array element and array elements immediately adjacent said resonating array element and to correspond to a phase shift of substantially 180 degrees between said end and twodiifths point and said two-fifths and four-fifths point of said resonating array element.

19. A log-periodic antenna comprising: a plurality of array elements arranged and dimensioned to form a logperiodic array of elements outwardly inclined in the direction yof radiation; frequency generator means; and a iirst and second feed conductor connected across oppositely poled output terminals of said generator means and alternately to the mid-points of successively larger array elements, said feed conductors being arranged and interconnected with alternating array elements to provide sufficient loading to correspond to a phase shift greater than degrees between a resonating array element and arnay elements immediately adjacent said resonating array element.

i120. A log-periodic antenna restrictively excitable in the three-half mode, said antenna comprising: a plurality of array elements arranged and dimensioned to -form a logperiodic array of elements on opposite sides of an array axis and outwardly inclined in the direction of radiation; a frequency generator tmeans; and first and second feed conductors connected between a pair of oppositely poled output terminals fof said generator and transposed between adjacent end points of array elements on opposite sides of said array axis, third and fourth feed conductors connected between said output terminals of said ygenerator means and alternately to the two-thirds point of the array elements on one side of said array axis, and fifth and sixth feed conductors connected between said output terminals and alternately to the two-thirds point of the array elements on the other side of said array axis, said feed conductors being arranged and interconnected 'with said array elements to provide sufficient loading to corres-pond to a phase shift greater than 180 degrees between a resonating array element and every second array element immediately adjacent said resonating element and to correspond to a phase shift of substantially 180 degrees between said end and twothird point of said resonating element.

2,1. A log-periodic antenna useful for forming an upward directed beam for ionospherical reflection to a distant point, said antenna comprising: a pair of planarly extending component antennas of mirror symmetry arranged to form a dihedral antenna lof a selected dihedral angle; means for supporting said dihedral antenna above the ground so that the apex of said dihedral antenna has a selected inclination with respect to the ground; and each component antenna including a plurality of array elements arranged and dimensioned to form a log-periodic array of inclined array elements and further including Lfeed means coupled to each array element for restricting excitation of each component antenna to a selected combination of modes, the lowest mode being equal to or greater than the full wavelength mode of a resonant array element.

22. A log-periodic antenna in accordance with claim ll in which said component antennas are excited by a common signal means.

References Cited the Examiner UNITED STATES PATENTS 2,964,748 12/1960 Radford 343-7925 X 2,989,749 i6/1961l Du Hamel et al. 343-806 X 3,056,960 l0/l962 Wiokersham 343-7925 3,086,206 4/1963 Greenberg 343-908 X ELI LIEBERMAN, Acting Primary Examiner.

HERMAN KARL SAALB-ACH, Examiner. 

1. A LOG-PERIODIC ANTENNA COMPRISING: A PLURALITY OF ARRAY ELEMENTS ARRANGED AND DIMENSIONED TO FORM A LOGPERIODIC ARRAY OF ELEMENTS INCLINED WITH RESPECT TO AN ARRAY AXIS; EACH OF SAID ARRAY ELEMENTS COMPRISING AN ELONGATED RADIATING CONDUCTOR; AND FEED MEANS COUPLED TO EACH OF SAID ELEMENTS FOR RESTRICTING EXCITATION OF SAID ARRAY TO A SELECTED COMBINATION OF MODES, SAID FEED MEANS INCLUDING A FEED CONDUCTOR COUPLED TO AN ELONGATED RADIATING CONDUCTOR AT A LOCATION BETWEEN ITS ENDS DETERMINED BY THE RADIATION MODE, SAID FEED CONDUCTOR MAKING AN ANGLE WITH SAID ELONGATED RADIATING CONDUCTOR TO PRODUCE A PREDETERMINED PHASE SHIFT. 